A switching power supply circuit has better linear characteristic, consumes low power, transforms electrical power efficiently, and works steadily. Therefore, the switching power supply is commonly used in an LCD TV, the display device, and other consumer products.
FIG. 2 shows a typical switching power supply circuit. The switching power supply circuit 10 includes a first rectifying filtering circuit 11, a first transformer 12, a second rectifying filtering circuit 13, a first direct-current (DC) voltage output port 136, a third rectifying filtering circuit 14, a second DC voltage output port 146, a first switching control circuit 15, a first feedback circuit 161, a second transformer 18, a fourth rectifying filtering circuit 17, a third DC voltage output port 176, a second switching control circuit 19, and a second feedback circuit 162.
The first rectifying filtering circuit 11 includes a full bridge-rectifier circuit 111, a first filtering capacitor 112, two input terminals 113, 114, and an output terminal 115. The two input terminals 113, 114 receive an external alternating current (AC) voltage, and the full bridge-rectifier circuit 111 transforms the AC voltage into a DC voltage. The first filtering capacitor 112 has one end grounded, and the other end connected to the output terminal 115.
The first transformer 12, including the first primary winding 121 and the first secondary winding 122, receives the DC voltage and outputs a first induced voltage and a second induced voltage. The first primary winding 121 has a terminal “a” and a terminal “b”, in which the terminal “a” is connected to the output terminal 115 of the first rectifying filtering circuit 11, and the terminal “b” is grounded through the first switching control circuit 15. The first secondary winding 122 includes a terminal “c”, a terminal “d”, and a terminal “e”, in which the terminal “c” is connected to the second rectifying filtering circuit 13, the terminal “d” is connected to the third rectifying filtering circuit 14, and the terminal “e” is grounded.
The first switching control circuit 15 includes a first pulse width modulation (PWM) chip 151, a first transistor 152, and a first resistor 154. The PWM chip 151 includes a first voltage sampling terminal 153 and a first control terminal 155. The first voltage sampling terminal 153 receives a feedback signal. The first control terminal 155 provides a first pulse signal to a gate of the first transistor 152. A source of the first transistor 152 is grounded through the first resistor 154, and a drain of the first transistor 152 is connected to the terminal “b” of the first primary winding 121. The first transistor 152 is an N channel metal oxide semiconductor field effect transistor (NMOSFET) of enhanced type.
The second rectifying filtering circuit 13 outputs a first DC voltage, in which the second rectifying filtering circuit 13 includes a first inductor 131, a first storage capacitor 132, a second filtering capacitor 135, a first rectifying diode 133, and a second rectifying diode 134. Anodes of the first rectifying diode 133 and the second rectifying diode 134 are connected to the terminal “c” of the first secondary winding 122, and cathodes of the first rectifying diode 133 and the second rectifying diode 134 are connected to the first DC voltage output port 136. The first inductor 131 has one end serially connected to one end of the first storage capacitor 132, and the other end connected to the terminal “c” of the first secondary winding 122. The other end of the first storage capacitor 132 is connected to the first DC voltage output port 136. The second filtering capacitor 135 having one end connected to the first DC voltage output port 136, and the other end grounded.
The third rectifying filtering circuit 14 outputs a second DC voltage, in which the third rectifying filtering circuit 14 includes a second inductor 141, a second storage capacitor 142, a third filtering capacitor 145, a third rectifying diode 143, and a fourth rectifying diode 144. Anodes of the third rectifying diode 143 and the fourth rectifying diode 144 are connected to the terminal “d” of the first secondary winding 122, and cathodes of the third rectifying diode 143 and the fourth rectifying diode 144 are connected to the second DC voltage output port 146. The second inductor 141 has one end serially connected to one end of the second storage capacitor 142, and the other end connected to the terminal “d” of the first secondary winding 122. The second storage capacitor 142 has the other end connected to the second DC voltage output port 146. The third filtering capacitor 145 has one end connected to the second DC voltage output port 146, and the other end grounded.
The first feedback circuit 161 receives the first DC voltage and the second DC voltage, generates a corresponding first feedback signal, and feedbacks the first feedback signal to the first voltage sampling terminal 153 of the PWM chip 151. The first feedback circuit 161 includes a first feedback input terminal 163, a second feedback input terminal 165, and a first feedback output terminal 167. The first feedback input terminal 163 is connected to the first DC voltage output port 136, and the second feedback input terminal 165 is connected to the second DC voltage output port 146. The first feedback output terminal 167 is connected to the first voltage sampling terminal 153 of the PWM chip 151.
The second transformer 18 receives the DC voltage and outputs a third induced voltage, in which the second transformer 18 includes a second primary winding 181 and a second secondary winding 182. The second primary winding 181 includes a terminal “o” and a terminal “p”, in which the terminal “o” is connected to the output terminal 115 of the first rectifying filtering circuit 11, and the terminal “p” is grounded through the second switching control circuit 19. The second secondary winding 182 includes a terminal “r” and a terminal “s”, in which the terminal “r” is connected to the fourth rectifying filtering circuit 17, and the terminal “s” is grounded.
The second switching control circuit 19 include a second PWM chip 191, a second transistor 192, and a second resistor 194. The second PWM chip 191 includes a second voltage sampling terminal 193 and a second control terminal 195. The second voltage sampling terminal 193 receives a feedback signal. The second control terminal 195 provides a second pulse signal to a gate of the second transistor 192. A source of the second transistor 192 is grounded through the second resistor 194, and a drain of the second transistor 192 is connected to the terminal “p” of the second primary winding 181. The second transistor 192 is an NMOSFET.
The fourth rectifying filtering circuit 17 outputs a third DC voltage, in which the fourth rectifying filtering circuit 17 includes a third inductor 171, a third storage capacitor 172, a fourth filtering capacitor 175, a fifth rectifying diode 173, and a sixth rectifying diode 174. Anodes of the fifth rectifying diode 173 and the sixth rectifying diode 174 are connected to the terminal “r” of the second secondary winding 183, and cathodes of the fifth rectifying diode 173 and the sixth rectifying diode 174 are connected to the third DC voltage output port 176. The third inductor 171 has one end serially connected to one end of the third storage capacitor 172, and the other end connected to the terminal “r” of the second secondary winding 182. The other end of the third storage capacitor 172 is connected to the third DC voltage output port 176. The fourth filtering capacitor 175 has one end connected to the third DC voltage output port 176, and the other end is grounded.
The second feedback circuit 162 receives the third DC voltage, generates a corresponding second feedback signal, and provides the second feedback signal to the second voltage sampling terminal 193 of the second PWM chip 191. The second feedback circuit 162 includes a third feedback input terminal 164 and a second feedback output terminal 166. The third feedback input terminal 164 is connected to the third DC voltage output port 176, and the second feedback output terminal 166 is connected to the second voltage sampling terminal 193 of the second PWM chip 191.
The switching power supply circuit 10 operates as follows:
The external AC voltage is inputted to the input terminals 113, 114 of the first rectifying filtering circuit 11, and is transformed into the DC voltage by the first rectifying filtering circuit 11. The DC voltage is then provided to the first primary winding 121 and the second primary winding 181 of the first transformer 12 and the second transformer 18 respectively. The first control terminal 155 of the PWM chip 151 outputs the first pulse signal for controlling a conduction of the first transistor 152, through which a current flowing through the first primary winding 121 is controlled, and a working state of the first transformer 12 is controlled as a result. The second control terminal 195 of the second PWM chip 191 outputs the second pulse signal for controlling a conduction of the second transistor 192, through which a current flowing through the second primary winding 181 is controlled, and a working state of the second transformer 18 is controlled as a result.
When the first transistor 152 is conducted, the first filtering capacitor 112, the first primary winding 121, the first transistor 152, and the first resistor 154 form a loop, thereby a first conducting current can flow through the first primary winding 121.
When the first transistor 152 is cut off, the first conducting current is consumed and becomes lesser, which induces a first induced magnetic field on the first primary winding 121. Influenced by the first induced magnetic field, the first induced voltage is generated on the terminal “c” of the first secondary winding 122, and the second induced voltage is also generated on the terminal “d” of the first secondary winding 122.
The first induced voltage is provided to the second rectifying filtering circuit 13. The first induced voltage is transformed into the first DC voltage through rectified and filtered by the second rectifying filtering circuit 13. The second induced voltage is provided to the third rectifying filtering circuit 14. The second induced voltage is transformed into the second DC voltage through rectified and filtered by the third rectifying filtering circuit 14. In addition, the first DC voltage and the second DC voltage from the first DC voltage output port 136 and the second DC voltage output port 146 are provided to the load (not shown). Furthermore, the first DC voltage and the second DC voltage are inputted to the first feedback circuit 161. The first feedback circuit 161 outputs the first feedback signal according the first DC voltage and the second DC voltage to the first voltage sampling terminal 153 of the PWM chip 151. The first PWM chip 151 regulates the first pulse signal according to the first feedback signal, so that the first DC voltage and the second DC voltage are also regulated accordingly, in which the first DC voltage is 24V, and the second DC voltage is 12V.
Similarly, when the second transistor 192 is conducted, the first filtering capacitor 112, the second primary winding 181, the second transistor 192, and the second resistor 194 form a loop, thereby a second conducting current can flow through the second primary winding 181.
When the second transistor 192 is cut off, the second conducting current is consumed and becomes lesser, which induces a second induced magnetic field on the second primary winding 181. Influenced by the second induced magnetic field, the third induced voltage is generated.
The third induced voltage is provided to the fourth rectifying filtering circuit 17. The third induced voltage is transformed into the third DC voltage through rectified and filtered by the fourth rectifying filtering circuit 17. In addition, the third DC voltage from the third DC voltage output port 176 is provided to a load (not shown). In addition, the third DC voltage is inputted to the second feedback circuit 162. The second feedback circuit 162 outputs the second feedback signal to the second PWM chip 191 according to the third DC voltage. The second PWM chip 191 regulates the second pulse signal according to the second feedback signal, so that the third DC voltage is also regulated accordingly, in which the third DC voltage is 5V.
As described above, the switching power supply circuit 10 controls the transformer 12 and the transformer 18 through the first PWM chip 151 and the second PWM chip 152 to get the first DC voltage, the second DC voltage, and the third DC voltage. However, the first PWM chip 151 and the second PWM chip 152 are the same type, which may cause the electromagnetic interference effect, and the stability of the switching power supply circuit 10 is affected as a result. Besides, the transformer 12 and the transformer 18 are heavy, which also makes the switching power supply circuit 10 heavy. Further, because the first PWM chip 151 and the second PWM chip 152 are costly, the cost of the switching power supply circuit 10 is increased as a result.